In April this year, seven miners were rescued by the underground rescue unit after a fire broke out at Boulby Mine.

But ICL insisted the site was still viable, and has applied to extend its licence to mine at Boulby for another 40 years, by switching from potash to the extraction of another fertiliser, Polysulphate.

Its current licence runs out in 2023, and an extension requires approval from the North York Moors Park Authority.

Peter Smith, the firm’s executive vice president for potash, said the Boulby plant would be “restructured” around the extraction of Polysulphate, while its limited remaining reserves of potash would continue to be mined at a reduced rate until exhausted.

He said Boulby would remain “a significant employer in the area”, but added: “When we announced the restructuring of the business last November we made it clear that, given the very limited level of economically feasible potash reserves, we had to move our focus to Polysulphate production.”

Mr Smith said the company was developing a range of fertiliser products based on Polysulphate, as well as a compacted potash and Polysulphate product to be marketed as Potashplus.

He added: “The restructuring, although difficult, is aimed at ensuring that ICL UK has a future as a supplier of world-class products for many years to come, which is underlined by our decision to submit an application to the North York Moors Park Authority to extend our planning approval for a further 40 years.”

Redcar’s Labour MP Anna Turley called the latest job losses “disappointing news for the local economy”.

She said: “Whilst ICL UK have been clear that reductions to the workforce would be made by 2018 when supplies of their traditional potash fertiliser stock are expected to completely run out, it is nevertheless unwelcome news for the local economy, which has already sustained a large number of job losses in the last year.”

However, Ms Turley said the planned Teesport Renewable Energy Plant near Middlesbrough, and a projected £2.4bn potash mine to be built near Whitby by Sirius Minerals, would both create new job opportunities in the area.

Boulby, which employs around 1,000 people, has been hit by a series of tragedies since last November’s redundancies.

In June, a worker was killed in an underground explosion while the plant was on a state of alert over possible gas escapes. John Anderson, 56, died when a sudden release of gas occurred in the section where he was working.

Two weeks ago, a contractor was airlifted to hospital after suffering burns in an incident at the site.

Four months earlier, seven miners were rescued after a fire broke out in supposedly fire-retardant polystyrene blocks used to circulate air around the mine.

And in 2014, the Mines Inspectorate was called in after an underground roof collapse.

ICL insisted that safety at the site was a “top priority”.

Boulby is the world’s only producer of Polysulphate, which is claimed to be more environmentally friendly than other fertilisers and contains four plant nutrients of sulphur, potassium, magnesium and calcium.